1. Field of the Invention
The present invention relates to radio communication system and method for providing broadband packet communications with respect to subscribers moving by automobiles, etc.
2. Description of the Related Art
In conjunction with explosive spread of the Internet, it has become feasible to make accesses to images or applications such as execution files through the Internet not only from indoor environments such as home or office but also from outdoor environments such as street or inside a moving automobile. In particular, a service for downloading files containing music, video, advertisement information, etc., through the Internet is expected to be the major way for utilizing the Internet in the outdoor environments.
In the case of utilizing the Internet from the outdoor environment, a subscriber will access the Internet by utilizing a mobile communication system such as that of mobile phone or cellular phone. However, the current transmission bandwidth (about several tens Kbit/sec.) of the mobile communication system is hardly sufficient for comfortably receiving the service through the Internet.
There are also discussions of a next generation radio access system such as IMT-2000 (International Mobile Telecommunications 2000) and MMAC (Multimedia Mobile Access Communication). However, in the IMT-2000, it is possible to realize the communication speed of 2 Mbit/sec. while at rest, but the maximum speed that can be provided during a fast moving by an automobile is only 144 Kbit/sec. Also, the MMAC system can provide the transmission speed of 10 Mbit/sec. per user, but it can only handle a moving speed at a level of the walking speed so that it is impossible to receive services during a fast moving by an automobile.
In addition to such a shortage of the transmission bandwidth at the radio access section, the handover for switching an access target radio base station for a moving subscriber can also be a source of degradation for the performance of the internet access utilizing the mobile communication system. Namely, when the handover occurs, it is necessary to carry out a procedure for detecting a new access target radio base station, a procedure for establishing connection with this new access target radio base station, a procedure for terminating connection with the previous access target radio base station, as well as a management of the subscriber's move for the purpose of continuing the data transmission to the subscriber.
In the case where the subscriber wishes to receive the downloading service through the Internet, a data server that is the downloading source cannot recognize the radio base station to which the downloading data destined to that subscriber should be sent, until the connection with the new access target radio base station is established. Namely, only after the handover is finished, the data server can recognize the new access target radio base station and restart the transmission of the downloading data to the subscriber through that radio base station if the transmission has not been completed yet. At this point, the time available for the data transmission using the radio base station will become shorter as much as the time required for the handover control becomes longer, and this can cause the lowering of the data transmission speed.
As described above, considering the fact that the standard communication speed in the current mobile communication system and the next generation radio access system currently under the discussions is 10 Mbit/sec. in the wired LAN, the communication speed under the fast moving environment is hardly sufficient for comfortably receiving services through the Internet. Also, the handover control to be executed in conjunction with the occurrence of the handover can be a cause for the lowering of the data transmission speed.